I would go with 3 shaman if it were me, or warlocks or something.

Mages seem to be pretty reliant on mobility, mostly through blink, which is difficult to use as a multiboxer (unless you wanna blink them all at the same time). Then again, I haven't boxed mages, so I wouldn't know. All I know is shaman hit almost as hard, wear mail + shield, and being an offensive hybrid they have a myriad of abilities that work particularly well when stacked.